1. Field of the Invention
The present invention relates to a filter circuit and, more particularly, to a switched-capacitor filter circuit.
2. Description of the Related Art
In the audio field and various other communication fields, a digital-to-analog (D/A) converter is used to output an analog signal in response to input of a bit stream digital signal. An analog signal output from such a D/A converter is input to a low pass filter (LPF) formed using a switched capacitor and a differential amplifier. The analog signal input to the LPF is filtered to output the filtered signal as an analog signal having sufficiently reduced harmonics components.
Japanese Patent Laid-Open Nos. 5-075471 and 10-070466 (patent literatures 1 and 2) disclose the following configuration. That is, patent literatures 1 and 2 provide a filter circuit in which a 1-bit D/A converter in the foregoing stage is formed by a switched capacitor and a fully-differential amplifier and receives a bit stream digital signal, and a D/A converter in the subsequent stage is connected to an LPF formed by a switched capacitor and a fully-differential amplifier.
The fully-differential amplifier includes a common-mode feedback circuit (to be abbreviated as a CMFB circuit hereinafter) to stabilize the direct current (DC) level of an output. The CMFB circuit performs feedback so that the average voltage value of two signals output from the fully-differential amplifier becomes a predetermined voltage value. There are two types of CMFB circuits: continuous- and discrete-time type CMFB circuits. The continuous-time type CMFB circuit includes a feedback circuit which matches an average voltage value obtained by dividing the two output signals by the resistances with a predetermined voltage value.
A fully-differential amplifier which uses a switched-capacitor circuit typically employs a discrete-time type CMFB circuit. The discrete-time type CMFB circuit performs feedback discretely by, for example, alternately sampling two output signals and predetermined voltage values into a capacitor. However, patent literatures 1 and 2 give no details of the configuration of the CMFB circuit in the fully-differential amplifier.